Casey Brian

Southern Illinois University Carbondale graduate student Casey Bryan is participating in a study of various clovers as a flowering cover crop for efficient weed suppression, pollinator habitat and water quality enhancement. She and other SIU researchers are participating in a three-part study with funding from the USDA Forest Service and the U.S. Fish and Wildlife Service. Here, Bryan is netting pollinators in a clover test field in the Crab Orchard National Wildlife Refuge as part of the study. (Photo by Russell Bailey)

July 31, 2017

SIU Researchers explore weed suppression

by Andrea Hahn

CARBONDALE, Ill. – Everyone likes a win-win situation. Researchers from Southern Illinois University Carbondale are conducting studies at Crab Orchard National Wildlife Refuge to find the right formula for a win-win for farmers and insects that are pollinators. 

The idea is to manage weeds effectively using flowering cover crops that may create habitat and food resources for pollinator species. 

Karla Gage, an assistant professor in both the colleges of agricultural science and science, is a weed ecologist and plant biologist. She’s been focusing on Palmer amaranth and waterhemp, two species that spread rapidly, compete with soybeans and tend to be herbicide resistant. 

Gage is looking at bob oats and several species of clover – crimson, red and white, and also a mixture of clovers -- as potential weed-suppression, pollinator-friendly cover crops. 

Crab Orchard is a wildlife refuge where habitat is a priority, yet growers are able to harvest crops planted in accordance with the refuge goals. The crop rotation is corn, then soybean followed by a fallow season that is typically planted in winter wheat -- not a typical rotation for the area. The wheat is aerially seeded into standing soybeans in late summer and remains in the field for the following year’s growing season, termed the “rest year” or fallow season. Gage and the student researchers assisting with the project are taking advantage of the idiosyncrasies of Crab Orchard farming to plant the clovers instead of the winter wheat during the study. The immediate application of their work may help drive farming practices at Crab Orchard, but may also be of use to farmers seeking to control weeds and support natural ecosystems. 

Gage is interested in how the clovers and bob oats will interact with the existing weed community. She hopes the clover will thrive and keep the driver weeds -- the Palmer amaranth and water hemp -- at bay and contribute to the natural ecosystem without adversely affecting the crop yield. Driver weeds are those that drive weed management decisions. 

“Cover crops provide microbial diversity and organic matters in the soil,” Gage said. “Other advantages of cover crops include nutrient retention, drought resistance, and soil conservation by always having a living root in the field.” 

The research team has 25 fields in Crab Orchard -- about 330 acres total of the 4,000 or so in farm production. The wheat acts as a cover crop. However, the aerial seeding makes for a sparse crop that leaves plenty of room for weeds. The team thinks the clovers might do a better job with weed suppression than winter wheat and, as a floral resource, provide pollinator habitat. 

“No one is going to give up a growing season to plant clover over an entire field,” Gage said. “But if the clovers are effective in a weed management strategy, they may make good choices for field buffers and edges.” 

Casey Bryan, a graduate student in plant biology, is working closely with Gage at Crab Orchard. She conducted baseline weed surveys before this study began, and now she is collecting pollinators as part of a larger pollinator inventory. 

“Government agencies are tasked with managing for pollinators – but there is limited information of what pollinators occur across the landscape or what the life requirements of each of the pollinator species are,” she said. “We need to establish a baseline to inform land management practice.” 

She’s also interested in weed suppression, and she hopes the clovers study will show benefits of floral resource cover crops. 

“As herbicide resistant weeds increase, switching up crop rotation can make a difference,” she said. “The challenge is how it impacts the farmer’s bottom line. Many farmers know the benefits of cover crops, but they do require management. We’re hoping this study may give farmers another tool for weed suppression that comes with environmental benefits.” 

Jon Schoonover, professor of forestry, is conducting a water quality study in conjunction with the clover cover crop study. 

His student research team helped install two kinds of lysimeters for data collection. Pan lysimeters measure gravitational water leaching into the soil below the root level, and tension lysimeters pull water from the pore spaces in the soil to measure what is available to the plants. The team installed the tension lysimeters 18 inches deep. Now that the lysimeters are in place, the student researchers are taking water measurements and analyzing water samples for nitrogen content. 

“If the nitrate goes below the roots, it’s likely going into the groundwater and eventually to a stream,” Schoonover said. 

Legume cover crops, such as clovers, act as nitrogen fixers. They contain symbiotic bacteria -- rhizobia -- in root system nodules that help the plant produce nitrogen compounds. When the plant dies, it releases the fixed nitrogen, fertilizing the soil. The common crop rotation of corn after soybeans capitalizes on this natural fertilizer, though the possibility remains of nitrates leaching into the groundwater and waterways. 

“Because this is a pollinator study, we are leaving the clover on the fields rather than taking it off at planting time,” Schoonover said. “This will give us an idea of how the cover crop affects nitrogen leaching throughout the growing season if the clover cover crop is used for field borders. We know field borders are particularly important when they border a stream. Part of the study is to determine if the clover cover crops benefit pollinators, and our part is to determine the impacts that leguminous cover crops have on water quality.” 

The three-year grant is jointly funded by the U. S. Department of the Interior and the U.S. Department of Agriculture under the Service First Authority. Local partners are: USDA Forest Service, U.S. Fish and Wildlife Service and SIU. The study is in its first year.